This invention relates to a case hardening steel which is suitable for carburizing of relatively high temperatures.
The power trains in transportation machines represented by automobiles, other industrial machines or agricultural machines include various structural machine parts such as gears, bearings and shafts. In general machine structural carbon steels or alloy steels are employed as the material of such structural machine parts, and in many cases the machine parts fabricated by a suitable forming method are subjected to a surface hardening treatment such as gas carburizing or carbonitriding.
Conventional surface hardening treatments for this purpose are commonly carried out at temperatures below 950.degree. C. and consume a very long time to accomplish carburizing or carbonitriding to a required depth. Therefore, the surface hardening treatment has offered an obstacle to the enhancement of the productivity of the aforementioned machine parts.
With such a technological background, a vacuum carburizing method has been developed as one of recent carburizing techniques which are expected to enable to accomplish sufficient carburizing in a fairly short time. In general the vacuum carburizing treatment is carried out at relatively high temperatures and usually at temperatures above 950.degree. C.
However, the employment of higher temperatures in carburizing has offered new problems to the industrial production of the machine parts. When structural machine parts formed of a conventional machine structural carbon steel, which is usually a so-called case hardening steel, are subjected to the high temperature vacuum carburizing treatment, the high temperature of the treatment is liable to cause coarsening of the grain size of the treated steel so that the machine parts are liable to suffer from a great thermal strain or significant lowering in mechanical strength. As a countermeasure, it is usual to carry out a so-called grain refining treatment subsequently to the vacuum carburizing treatment. That is, the carburized machine parts are once cooled to a temperature below the transformation temperature and again heated up to the austenizing temperature and then quenched. However, the addition of the cooling and reheating process to the vacuum carburizing treatment means a considerable increase in the length of time required for accomplishment of the surface hardening, so that the total operation time does not become so short as expected compared with the conventional gas carburizing treatment. This is a major reason for the slowness of industrial popularization of the vacuum carburizing method which is advantageous in respect of the carburizing efficiency.
To solve the above described problems in the vacuum carburizing method it has been tried to develop a new steel which possesses an austenite-ferrite two-phase structure at the high temperatures employed in carburizing. However, the results have not been fruitful. Thus far, the researches have attained some success in grain refining of the core portion of a carburized steel body, but grain refining of the hardened case portion is still difficult.